This invention relates to magnetic levitation systems, and more particularly to using magnetic levitation in linear transport systems which transport materials between process stations in a vacuum environment.
The use of linear transport systems is well known throughout various industries. For example, many manufacturing operations utilize linear transport systems to move the materials being fabricated into and out of vacuum chambers, where the materials are treated. The use of magnetic levitation enables the transport mechanism to be substantially free of dust and other particulates. For example, Enomoto U.S. Pat. No. 5,288,199 describes a transporting device which uses a magnetically levitated floating arm to transport semiconductor wafers between multiple chambers of a wafer fabrication system. Enomoto relies on the use of proportional/integrating/differentiating (PID) control circuits to maintain the slider arm in the proper horizontal position and a judging circuit to determine whether a wafer is present on the arm.
In another system, Oshima et al. U.S. Pat. No. 5,241,912 describes a magnetically floating carrier in which electromagnets act through a reduced thickness portion of the wall of a lateral member. Oshima, like Enomoto is directed toward an apparatus for transporting semiconductor wafers in a vacuum chamber. Oshima utilizes a sealed tubular portion that is typically maintained at atmospheric pressure, while a carrier that slides along the tubular portion is disposed in the vacuum. A drive system is disposed within the tube that includes two pairs of magnetic bearings and various position sensors. The magnetic bearings interact with magnetic targets that are placed on the carrier to cause the carrier to laterally float back and forth. In order to reduce the power requirements of the system, Oshima discloses a technique whereby the thickness of certain portions of the outer wall of the tubular member (i.e., where the magnetic targets are placed) is reduced to improve the linkage between the bearings and the targets.
Another variation of a linear semiconductor transportation apparatus is disclosed by Belna U.S. Pat. No. 4,624,617. Belna's device utilizes a linear induction motor to levitate and linearly propel a car that supports a semiconductor wafer. The motor includes permanent magnets disposed on the car that interact with individually energizable electromagnets disposed along a track that the car travels along. The car is guided down the track by a pair of permanent magnet grooves (i.e., "V" shaped) in the underside of the car and pair of "V" shaped magnets that extend down from the track to the grooves. The electromagnets act to offset the natural magnetic forces between the grooves and the "V" shaped magnets such that the car levitates above the track. As the electromagnets are sequentially energized, the car moves linearly along the track.
While each of the above described devices may be adequate for transporting one or more semiconductor wafers, none of the devices are capable of transporting objects of greater weight, possibly even including cassettes holding multiple wafers. For example, Oshima describes how extra power would be necessary to transport single wafers without reducing the thickness of the tubular walls. The extra power, besides being a negative factor itself, may cause severe problems due to heat generation. In fact, the heat dissipation problem is particularly applicable in applications such as Belna in which the electromagnets may not be located in a region at atmospheric pressure.
In view of the foregoing, it is an object of this invention to provide improved magnetically levitated transport devices that operate with reduced power requirements.
It is also an object of this invention to provide improved magnetically levitated transport devices that are capable of transporting objects weighing more than a pound.